Added line decoration renderer.

[mkrueger]

I use the line decoration renderer to add preview to completion items.

For example:

This shows the preview of "it" when pressing tab the completion will extend the word to "exit".

[mkrueger]

@patriksvensson any feedback for that one?

[patriksvensson]

@mkrueger Sorry, I didn't realize this was an extension point, so I was waiting for the implementation. Looks good to me.

[mkrueger]

thanks - y my completion implementation is tied to the cmd line parser I use - it completes the syntax tree. Not sure if it's really worth providing an API for that. The basic thing is about ~150 LOC.
(without the parser - just the completion)